Generally, a coaxial cable is a transmission line including a central conductor for transmitting signals, and a shield coaxially formed on the central conductor. Seeing the inside section of the line, the central conductor and the shield are coaxially arranged, and an insulation layer having a dielectric feature is formed between the central conductor and the shield.
Various kinds of coaxial cables with various sizes have been developed, and such a coaxial cable is advantageous since attenuation of signal and change of propagation delay caused by frequency are small owing to its structural features, a large amount of data may be transmitted in a lump, and various coaxial cables may be received in the same cable while ensuring little leakage of signal among them.
An impedance characteristic is the most essential factor of the coaxial cable, and an impedance value is decided based on the following Equation 1. At this time, in the Equation 1, Z0 is a characteristic impedance, εr is a permittivity, d is a diameter of the central conductor, and D is an inner diameter of the shield.
                              Z          0                =                              138                                          ɛ                                  r                  ⁢                                                                                                                      ⁢          log          ⁢                                          ⁢                      D            d                                              Equation        ⁢                                  ⁢        1            
As seen from the Equation 1, factors determining a characteristic impedance includes a permittivity, a diameter of the central conductor, and a diameter of the shield. At this time, the permittivity is increased or decreased depending on the degree of foam of the insulation layer, and a propagation velocity is increased or decreased depending on the permittivity. Here, the propagation velocity satisfies the following Equation 2. At this time, in the following Equation 2, υp is a propagation velocity, εr,exp is a permittivity after foaming, εr,sol is a permittivity before foaming, ρexp is a density after foaming, and ρsol is a density before foaming.
                                          v            p                    =                      1                                          ɛ                                  r                  ⁢                                                                                                                            ⁢                                  ⁢                              ɛ                          r              ,              exp                                =                      co            ⁢                                                  ⁢                          log              ⁡                              (                                                                            ρ                      exp                                                              ρ                      sol                                                        ⁢                  log                  ⁢                                                                          ⁢                                      ɛ                                          r                      ,                      sol                                                                      )                                                                        Equation        ⁢                                  ⁢        2            
As seen from the Equation 2, a permittivity is lowered as the degree of foam is increased, and a propagation velocity is improved as the permittivity is lowered. That is to say, the loss characteristic depending on signal propagation is improved. At this time, as foam cells composed in an insulation layer after foaming have higher density and uniformity, the degree of foam is increased.
Meanwhile, to improve the substantial loss characteristic of the coaxial cable, the diameters of the central conductor and the shield should not be nearly increased in a generally used cable. That is to say, if a propagation frequency reaches a high level of several GHz, the coaxial cable is confronted with a limit of high frequency due to the TEM (Transverse Electro Magnetic) mode. In addition, in case the materials of the central conductor and the shield are substituted with metal having excellent conductive properties, its performance in comparison to a manufacture cost is inefficient.
Thus, a desirable solution for improving the loss characteristic of the coaxial cable is to improve permittivity and structure of the insulation layer.
Recent studies for coaxial cables are directed to improving a structure between a central conductor and a shield and thus improving propagation features in order to reduce an energy loss caused by signal propagation. U.S. Pat. No. 6,912,777 and U.S. Pat. No. 4,866,212 disclose a coaxial cable in which an air layer with a lowest permittivity is arranged to surround the central conductor. In addition, as shown in FIG. 1, a wrinkled shield 3 is provided to surround a central conductor 1 and a shield 2, thereby improving a loss characteristic according to signal propagation.
Also, U.S. Pat. No. 6,130,385, U.S. Pat. No. 4,965,412 and US 2003/0051897 disclose a technique for improving a loss characteristic according to signal propagation by providing a metal layer or a film layer deposited with metal, which excellently shields electromagnetic wave, to an inner or outer side of the shield.
In addition, JP 1997-141990, JP 1998-217484 and JP 2001-387541 disclose a technique for improving a loss characteristic according to signal propagation by providing a skin layer surrounding an outer circumference of an insulation layer.
The above conventional techniques improve a loss characteristic in consideration of diameter and material of the central conductor and the shield, but they are confronted with a limit of high frequency or insufficient in performance compared with a manufacture cost. Also, the conventional techniques have a problem that a propagation characteristic is deteriorated due to low density and uniformity of foam cells since foam cells have irregular sizes or lumps with each other. Moreover, a low degree of foam causes local differences of permittivity and unbalanced outer diameters of the coaxial cable, and it also acts as a limitation factor in making a coaxial cable with a large caliber.
Recently, studies for lowering a permittivity by foaming polymer material are frequently progressed, and many efforts are consumed for using a high frequency of several hundred MHz or several GHz as a usable frequency so as to propagate more information. Accordingly, it is an important issue to develop a polymer insulation layer with a low loss.